This invention relates to self-clinching fasteners and assembly thereof with thin sheet materials such as sheet metal.
Self-clinching fasteners for typical sheet metal have been developed particularly for applications in which welding or threading are undesirable as the primary means of securing the joint, such as the self clinching fasteners marketed under the trademark STRUX.sup..RTM.. For example, U.S. Pat. No. 3,967,669 describes fasteners having an enlarged head and a shank portion having a tapered extruding section of the type installed in a pre-punched hole in the sheet metal which may be smaller than the extruding section of the shank. The shank extruding section is inserted and deforms a portion of the periphery of the sheet metal aperture to resize the aperture, preferably resulting in a rim formation on the aperture with the aperture walls closely overlying the retaining groove in the fastener. As the fastener head is driven into the sheet metal the rim or aperture material is displaced radially inwardly to fill the annular retaining groove provided in the fastener shank below the head which produces a firm mechanical interlock. In addition, the fastener head may include a plurality of radially arranged projections which, when embedded in the sheet material, prevent relative rotation and provide high torque resistance between the fastener and the sheet metal, particularly when the fastener is internally threaded to accept a second threaded fastener driven into the clinched fastener in a subsequent mounting operation.
In applications requiring very thin sheet metal, it has become difficult to achieve sufficient cold-flow deformation of sheet metal to fill the fastener retaining groove to produce the desired secure interlock which will resist rotation and dislodgement or push out of the fastener for dependable performance. This disadvantage in using very thin sheet metal is entirely eliminated by the improved fastener assembly method and joint provided by the present invention.